1. Field of the Invention
The present invention generally relates to fluid pumps and more specifically relates to magnetically driven centrifugal pumps.
2. Related Art
Magnetically driven centrifugal pumps are well suited for pumping corrosive or hazardous fluids because they do not have shaft seals that can leak or wear out. Pumps of this type require some method of opposing the thrust load that is created by the impeller. Typically this load is taken by an axial bearing as described in U.S. Pat. No. 4,226,574. To accommodate periodic load opposite to the normal direction that occur in certain pumping conditions, an additional axial bearing containment shell be used as described in U.S. Pat. No. 6,443,710. Another method of opposing these axial loads is by using a thrust balancing system such as that described in by Klein in U.S. Pat. No. 6,135,728. Klein uses fluid pressure to balance the thrust forces and controls the fluid pressure with a ring on the impeller hub that creates a variable gap between the ring and the shaft.
There is an inherent difficulty in controlling the thrust forces with a thrust balancing valve. Since pumps need to operate over a wide range of pressures and flows, a thrust-balancing valve must also be able to control the pressure over a wide range of pressures. In order to achieve this type of control, a pressure differential must always exist across the valve. Therefore, there is a need to achieve the lowest possible pressure on the outlet side of the valve, while also maintaining the highest possible static pressure on the back side of the valve.
Rotation of the fluid in the area that is used to balance the thrust forces creates a pressure gradient that reduces the pressure differential across the thrust balancing valve. Klein requires that the fluid travel through channels in the rotating impeller to get to the thrust balancing valve. These rotating passages contribute to the lack of available pressure at the back side of the valve.
Another problem that is typical for magnetically driven pumps in general is the need to provide different impeller sizes. Since the impeller is typically permanently attached to the magnet carrier, the cost of the assembly is relatively high. This means that spare parts or alternate impellers that need to be stocked become a much more expensive inventory. Even when these components are made separately, they are typically secured together in a manner that is not easily disassembled (e.g., the attachment structure disclosed in U.S. Pat. No. 5,895,203). Therefore there is a need for an impeller and magnet carrier that be easily separated, but can still transmit the required torque.
Another aspect of magnetically driven pumps is the need to periodically replace wear rings. Although removable wear rings are described in U.S. Pat. No. 6,234,748, they require separate retainer rings that are not easily removed. A need exists for a simple means of replacing wear rings without retaining rings.